There are known in the art machines having granulating heads for cutting under water, composed basically of a chamber closed on one side by a die and traversed axially by a revolving shaft, to which are firmly secured cutting elements composed basically of shaped blades which, brought into contact with the die and set in rotation by the shaft, carry out the cutting of the material.
To this chamber, the plastics material to be granulated, which is in the softened state, is fed through the die. At the same time a strong current of water is caused to flow inside the sealed chamber, in a direction transverse to the axis of the shaft and of the die. The flow of water has the dual purpose of cooling the plastics material leaving the die to make it suitable for cutting into granules and, once the cutting has been carried out, of removing the granulated material from this chamber by bringing it into the next processing zone.
Because the cutting of the material must be carried out in contact with the die and the contact must be the most uniform and constant possible, it is usual, in the present state of the art, to subject the cutters to a preloaded thrust adapted for achieving the contact, the thrust being applied by various means, after the correct positioning of the cutters relative to the die has been achieved by means of suitable mechanical sliding and stopping means.
All the efforts of the prior art have, however, been directed towards assuring the thrust against the die even, for example, by introducing a compression spring against the die itself. It has, however, been found that the cutters when set in rotation and immersed in the water generate a hydrodynamic force substantially proportional to the square of the speed of rotation of the cutters, to the shape of the cutters themselves and to their number.
The hydrodynamic force has two components: a first component parallel to the die and responsible for the resistant torque on the cutters, and a second component perpendicular to the die, which induces a reaction force of the water itself, tending to exert a thrust on the blades towards the die, greater by about one order of magnitude than the thrust applied by the aforementioned prestressed spring.
Because the quality of cutting, the wear of the blades and, above all, the wear of the die, which is the most expensive component, are substantially dependent upon the contact force between the blades and the die, the technical problem arises of balancing, from one moment to the next, the axial component of the hydrodynamic force for the purpose of avoiding deterioration of the quality of cut and excessive wear of the die, by keeping only the constant prestressing force which, for some applications, could even be zero.